Octane 101 - Motorcycle Gas

What Is An Octane Number, And What Does It Mean? And What Octane Rating Do You Really Need?

John Lavina (February 2010)

What is an octane number, and what does it mean? And what octane rating do you really need? And what is that knocking sound that keeps coming out of your engine?!!  Seriously though, those types of questions are commonly asked by anyone when they first get started in motorsports, and rightly so.  Here are the answers.

What is Octane Rating?

The octane rating of fuel that you see at the gas pump is actually the Anti Knock Index number, which is a measurement of the anti-knocking properties of the fuel. It is the average of two octane numbers, the RON (Research Octane Number) and MON (Motor Octane Number).  The RON number determines the low to medium-speed knock while the MON number determines the high-speed and partial throttle heavy load knock characteristics. The higher the Anti Knock Index number the better the fuel is to resisting premature detonation, or engine knock. In other words, lower octane fuel is actually more explosive than higher-octane fuel.

What's the risk?

Abnormal combustion can cause knocking or pinging sounds to come from your engine. This means that the gas that you are using is either burning uncontrollably in the combustion chamber (often referred to as detonation) or the fuel is being ignited prematurely by other sources such as a glowing fragment of carbon in the cylinder instead of waiting for the spark from the spark plug (this is called pre-ignition).  Obviously, any explosion that occurs before the piston reaches the top of it's rotation is going to work against the engine, not for it.  So it is important to have a gasoline that will not burn either uncontrollably or prematurely.

Should I Be Buying Race Gas?

If you are racing, and have a modified motor with very a high compression ratio, you might find the need to go for a very high-octane fuel.  But like most things, wait until it poses a problem, and then correct the problem.  If you are like most people, you don't have a “factory bike”, you have a bike from the factory.  The engineers at the motorcycle factory have designed, built and tested your bike so it would run well on pump gas.  Stick with premium pump gas, like it says in the manual and you'll do great.